1. Field of the Invention
The present invention generally relates to lithography for semiconductor fabrication, and in particular, to utilizing a liquid medium through which a semiconductor substrate is illuminated during photolithography. More particularly, the present invention relates to an immersion lithography system and apparatus that employs membrane barriers for controlling exposure of system components to immersion liquid.
2. Background Information
Lithography, in the context of building integrated circuits (ICs) such as microprocessors and memory chips, is a highly specialized printing process used to put detailed patterns onto silicon wafers. An image containing the desired pattern is projected onto the wafer through a mask defining the pattern. Prior to light projection through the mask, the wafer is coated with a thin layer of photosensitive material called “resist.” The bright parts of the image pattern cause chemical reactions which cause the resist material to become more soluble, and thus dissolve away in a developer liquid; the dark portions of the image remaining insoluble. After development, the resist forms a stenciled pattern across the wafer surface which accurately matches the desired mask pattern. Finally, the pattern is permanently transferred onto the wafer surface in an etching process wherein, for example, a chemical etchant is used to etch the portions of the wafer surface not protected by resist.
With the image resolution of lithography as the limiting factor in the scaling of the IC devices, improvements in lithographic components and techniques is critical to the continued development of more advanced and compact ICs. The scaling limitation for optical lithography is expressed by the Rayleigh equation:
  W  =            k      ⁢                          ⁢      λ        NA  where k is the resolution factor, λ is the wavelength of the exposing radiation, and NA is the numerical aperture. NA may be determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens, as follows:NA=n sin α
where n is the index of refraction of the medium surrounding the lens and α is the acceptance angle of the lens.
Faced with problems and limitations relating to using shorter wavelength light sources, optical lithography developers have increasingly looked for ways of increasing the NA of lithography systems. Having low radiation absorption characteristics, air has traditionally been used as the transmitting medium. However, having an index of refraction n=1, air as the radiation medium presents a clear limit to the NA and consequently to the minimum scaling size. Immersion lithography, in which a liquid having a higher index of refraction is used as the medium, is therefore rapidly emerging as an important candidate for upcoming semiconductor lithography applications.
A number of practical issues to implementing immersion lithography remain, including maintaining a pure, non-obstructing transmission medium and compatibility of the tools and wafer with the liquid medium. Purified and degassed water, having a light absorption of 5% at working distances up to 6 mm and an index of refraction n=1.47, may be a suitable medium for immersion lithography. However, problems remain relating to the tendency to form bubbles during the scanning processing. The stage on a lithography exposure tool steps from location to location across a wafer scanning the reticule image for each field. To achieve high throughput, the stage must accelerate rapidly, move accurately to the next field location, settle, scan the image and then step to the next location all in a short period of time. A water medium is susceptible to forming micro-bubbles and nano-bubbles in the cavitation prone water layer near the moving surfaces. In addition to problems associated with maintaining purity of the liquid, immersion lithography requires substantial redesign of the stage for compatibility in a submerged liquid environment requiring significant re-engineering and adding to development costs.
An improved immersion lithography apparatus that addresses the foregoing problems is disclosed by the above-incorporated patent application “LIQUID-FILLED BALLOONS FOR IMMERSION LITHOGRAPHY”. The improved apparatus utilizes a liquid-filled balloon that is positioned between a semiconductor workpiece and a lithography light source. With both the balloon membrane and the enclosed liquid exhibiting good optical properties, the system NA is increased for enhanced resolution without the additional complexity and expense of maintaining a liquid in contact with the lithography equipment and workpiece.
However, it would be desirable to further improve conventional immersion lithography techniques while providing more efficient membrane contained immersion liquid lithographic techniques.